Are authorities closing in on the key players in a wider terror network? It's difficult to say.

Three men were charged in Belgium on Monday, accused of participating in the activities of a terrorist group. But investigators have been tight-lipped about the specific allegations against people who are in custody. And in the days since the attacks, a number of people were arrested and charged, only to be released later when investigators said the evidence against them wasn't conclusive.

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That's what happened on Monday, when authorities announced a man they'd identified as Faycal C. and charged with "terrorist murder" was being set free.

"It clearly is a setback," CNN terrorism analyst Paul Cruickshank said. "They clearly at a certain point, perhaps based on eyewitness reports, believed that he might have been at the airport, might have been involved. They clearly do not now believe that. .... So there's this manhunt that goes on."

Who was third airport bomber?

After a major terrorist attack, it's no surprise to see people picked up and then let go, said Philip Mudd, a former CIA official and CNN counterterrorism analyst.

"The police do not have time to surveil people over weeks in the case of an imminent threat, because they fear that there's going to be another attack. So they're going to bring everybody in, see who's dirty, and some of those people are going to be released," he said. "This is pretty common in an investigation of this magnitude."

But one thing about the Brussels investigation so far, he said, is notable: We still don't know who the third alleged airport bomber was.

"The lack of identification to me is surprising," he said, "especially now that the public has seen this for so long. Somebody out there ... knows who he is."

Authorities in Belgium and the United States have a list of people who they believe he could be, and they're following leads from multiple places, law enforcement officials told CNN.

A young girl looks out of the window of a bus after airport evacuations.

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Photos:Terror attacks in Brussels

People stand near the airport after evacuations.

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Passengers gather outside the airport.

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Police officers stand guard near the Maelbeek metro station.

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A police helicopter flies above the area near the subway station.

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People embrace outside the Brussels airport.

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People are led away from the airport after the attacks.

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A victim receives first aid near the Maelbeek metro station.

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Ambulances arrive at the airport.

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But the task is complicated by the continuing manhunt, complete with raids, gunfire, explosions and live news conferences.

And at the airport, where two bombings significantly damaged the facility, officials said they're trying to get things up and running -- but more work needs to be done. On Tuesday, hundreds of airport staffers will conduct tests of a temporary setup for security screenings, checking in and checking luggage, the airport said in a statement Sunday. But it's too soon to say when the airport will reopen.

"The simple fact," the statement said, "is that a restart in the short term is not possible in the devastated infrastructure."